This invention relates generally to electrophotographic imaging members, and more specifically, to photoconductive imaging members with improved sensitivity characteristics obtained, for example, by the addition to the charge transport layer of hydroxy derivatives of charge transport molecules.
Electrophotographic photoreceptors are known and typically include a photoconductive layer formed on a conductive substrate. The photoconductive layer can be considered an insulator in the dark thus electric charges can be retained on its surface, however, upon exposure to light the charge is dissipated.
A latent image is formed on the photoreceptor by first uniformly depositing electric charges over the surface of the photoconductive layer by one of any means known in the art. The photoconductive layer acts as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity or countercharge on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the photoconductive surface not exposed to light retain their surface charge with the quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming a latent electrostatic image.
The photodischarge of the photoconductive layer usually requires this layer to photogenerate conductive charge and to transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors which perform both functions. These layers are laminated onto a conducting substrate and may include an optional charge blocking and an adhesive layer between the conducting and the photoconducting layers. Additionally, the substrate may be comprised of a nonconducting mechanical support with a conductive layer. Other layers to provide special functions, such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface, may be employed.
An example of a photoreceptor is a multilayered device comprised of a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example, in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. Other known charge transport molecules include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Another type of charge transport layer has been developed which utilizes a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a pendant or may form the backbone of the polymer. This type of charge transport polymer includes materials, such as poly(N-vinylcarbazole), polysilylenes, and others, including those described in U.S. Pat. Nos. 4,618,551; 4,806,443; 4,806,444; 4,818,650; 4,935,487 and 4,956,440, the disclosures of which are totally incorporated herein by reference.
Charge generator layers employed include amorphous films of selenium and alloys of selenium and arsenic, tellurium, germanium and the like; hydrogenated amorphous silicon and compounds of silicon and germanium, carbon, oxygen, nitrogen and the like, fabricated by vacuum evaporation or deposition; inorganic pigments of crystalline selenium and its alloys, III-V and II-VI compounds and organic pigments such as quinacridones, metal phthalocyanines, metal free phthalocyanines, polycyclic pigments such as dibromo anthranthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos, and the like dispersed in a film forming polymeric binder and fabricated by solvent coating.
Phthalocyanines have been employed as photogenerating materials for use in laser printers with infrared exposures. Infrared sensitivity is believed needed for low cost semiconductor laser diodes used as the light exposure source. Many metal phthalocyanines have been reported and include oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine.
One of the design criteria in the selection of the pigment for the generator layer and the selection of charge transporting molecule in the transport layer is that when light photons photogenerate holes in the pigment they be efficiently injected into the transporting molecule in the transport layer, that is, the injection efficiency from the pigment to the transport layer should be high. A second design criterion is that the injected holes be transported across the transport layer in a short time; shorter, for example, than the time duration between the exposure and development stations. The transit time across the transport layer can be determined by the charge carrier mobility in the transport layer, which charge carrier mobility is the velocity per unit field and has dimensions of cm.sup.2 /volt second. The charge carrier mobility is primarily a function of the structure of the charge transporting molecule, it's concentration in the transport layer and the "inactive" binder polymer in which the charge transport molecule is dispersed. It is believed that the injection efficiency can be maximized by selecting a transport molecule whose ionization potential is lower than that of the pigment, however, low ionization potential molecules may have other deficiencies, one of which is their instability in an atmosphere of corona effluents. Therefore, the sensitivity of the layered devices is primarily determined by three factors: (1) photogeneration efficiency; (2) injection efficiency from the pigment into the transporting medium surrounding it; and (3) the transport of the carriers through the transport layer. Pigments can be classified into: (1) intrinsic and (2) extrinsic. With intrinsic pigments, the charge transporting molecule which surrounds the pigment is not involved in the photogeneration step, while with extrinsic pigments, in the absence of the charge transporting molecule, the photogeneration efficiency is small; in the presence of the charge transport molecule, the photogeneration efficiency is higher. With a given pigment and a charge transport molecule, several factors enter into optimizing the sensitivity of the layered devices of the prior art, including: (1) pigment concentration and thickness of the generator layer; (2) the binder polymer in which the pigments are dispersed; (3) the procedure of milling, and the like employed to prepare the generator slurry; and (4) the transport layer thickness. The generator layer binder selection is also based on the ability to obtain dispersions and is sometimes selected on the basis of its manufacturability. Higher sensitivities can result in reduced laser power requirements and provide for an increase in the throughput of the xerographic process.
With the present invention, in embodiments significant increase, by as much as a factor 2, in sensitivity can be realized by the doping of the transport layer with, for example, small effective concentrations of transport molecules containing hydroxy groups, and other advantages can be achieved as indicated herein.